Magnetic vector type spectrometers having a mass-dispersive magnetic field are broadly classified into two major categories: the magnetic scanning type using a single ion detector and providing a mass spectrum by scanning the magnetic field; and the simultaneous detection type which uses a one or two-dimensional ion detector, such as an array detector, having spatial resolution and simultaneously detects analyte ions dispersed according to mass to charge ratio by the magnetic field.
Many of the mass spectrometers developed heretofore are scanning type mass spectrometers. The simultaneous detection type is theoretically superior in sensitivity to the scanning type because the former type detects all analyte ions simultaneously, while the latter type discards ions other than ions reaching the ion detector. However, one or two-dimensional ion detectors presently available are only photographic plates having low sensitivity and, therefore, simultaneous detection type mass spectrometers have not been widely accepted into general use.
As the resolution and the sensitivity of one or two-dimensional ion detectors have been improved by the introduction of advanced semiconductor fabrication techniques, the simultaneous detection type mass spectrometer which has excellent characteristics in principle has attracted attention in these years. In recent years, simultaneous detection has been attempted by combining various mass spectrometers with one or two-dimensional ion detectors. Such mass spectrometers are disclosed, for example, in the U.S. Pat. Nos. 4,435,642, 4,472,631, and 4,638,160.
Normally, a one or two-dimensional ion detector detects ions existing in a plane, which is hereinafter referred to as the "detection plane". On the other hand, in a simultaneous detection type mass spectrometer, analyte ions are dispersed according to mass toward a focal plane. This focal plane is a curved plane except where the ion optical system is a special ion optical system such as the Mattauch-Herzog geometry. FIG. 4 shows the relation among a mass analyzer 1 having a magnetic field, a one or two-dimensional ion detector 2, and a focal plane 3. As can be seen from this figure, the focal plane 3 is coincident with the detection plane 4 of the detector for ions of mass m.sub.2, and these ions are sharply focused onto one of the detecting elements constituting the two-dimensional detector. However, both planes do not agree for other ions of different masses such as masses m.sub.1 and m.sub.3. Ions of masses m.sub.1 and m.sub.3 impinge on the detection plane in defocused condition. In this geometry, the resolution deteriorates at the ends of the detector 2. For this reason, only a narrow central region of the spectrum can be observed. It is inevitable, therefore, that the measured mass range is narrow.